The bacteria that cause anthrax, diphtheria, and botulism are not closely related, but the sometimes deadly toxins they release share important features. NIAID grantee John Murphy, Ph.D., and colleagues at Boston University School of Medicine have defined one of the earliest steps in the process whereby these protein toxins enter and damage cells. They’ve also produced strong evidence that entry of at least two of the three toxins into cells requires interaction with the same human protein complex. Most importantly, their studies suggest a way to stop the toxins in their tracks.
To do damage, these three toxins must first bind to receptors on the cell surface, and then be ferried into the cell via a cellular transporter called an endosome. Release of the active portion of each toxin from the endosome and into the cell requires interactions between the toxin and one or more proteins in the cell’s cytoplasm. Determining exactly which of the many thousands of cell proteins are required for the toxins to complete their passage into the cell was a problem Dr. Murphy’s group worked on for several years.
In 2003, Dr. Murphy and his coworkers improved upon a way to halt trafficking of diphtheria toxin just before it emerged from the endosome. Using this refined method, the researchers pinpointed several human proteins in the cytosolic translocation factor (CTF) complex and showed that diphtheria toxin must interact with these proteins to get out of the endosome and into the cell.
In 2005, Dr. Murphy and his colleagues extended these findings by showing that a small bit of the diphtheria toxin interacted with a newly identified component of the CTF complex. Moreover, this interactive part of diphtheria toxin is identical to parts of both anthrax toxin and botulinum neurotoxin. This shared protein portion is termed a conserved motif. The implication: All three toxins exit the endosome in the same way—using the conserved motif—and all must bind to the same part of the CTF complex before cell damage can occur.
Most recently, the Boston scientists further clarified the picture of toxin entry into the cell by focusing on a part of anthrax toxin called lethal factor. The team found that anthrax lethal factor uses a mechanism of entry that is analogous to that of diphtheria toxin and, like diphtheria toxin, anthrax binds to a part of the CTF complex called the COPI (pronounced cop one) coatomer complex. Using the system they have perfected, the team is now testing the hypothesis that the interaction between the COPI complex and the conserved motifs of anthrax lethal factor and diphtheria toxin represent one of the earliest steps—perhaps the first one—in the toxins’ entry into the cell and the launch of their destructive effects.
Can the release of toxin from the endosome be blocked? Early research by Dr. Murphy and his colleagues, conducted on cells in test tubes, suggests the answer is yes. When the scientists added small, lab-created proteins—peptides—designed to bind to the same part of the COPI complex as is targeted by the toxins, the interaction was severely hampered. In effect, Dr. Murphy says, the peptides act as intracellular antitoxins. All antitoxin antibodies now in use, he notes, can block only the binding of the toxin to its cell surface receptor from the outside of the cell. Once the toxin binds to its receptor, there is no way to stop it from entering the cell. Peptides that could halt the exit of toxins from the endosome would provide an entirely new way to stop a toxin in its tracks. Further research to test this approach is under way in Dr. Murphy’s laboratory and elsewhere.
Ratts R et al. The cytosolic entry of diphtheria toxin catalytic domain requires a host cell cytosolic translocation factor complex. The Journal of Cell Biology DOI: 10/1083/jcb.200210028 (2003).
Ratts R et al. A conserved motif in transmembrane helix 1 of diphtheria toxin mediates catalytic domain delivery to the cytosol. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0504937102 (2005).
Tamayo AG et al. COPI coatomer complex proteins facilitate the translocation of anthrax lethal factor across vesicular membranes in vitro. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.080710100105 (2008).
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Last Updated February 18, 2009